Ex vivo B-cell cultures are important tools in current biological and medical applications. One important application is culturing antibody-producing cells in order to harvest antibodies, preferably monoclonal antibodies. Monoclonal antibodies (mAbs) represent multiple identical copies of a single antibody molecule. Amongst the benefits of mAbs is their specificity for the same epitope on an antigen. This specificity confers certain clinical advantages on mAbs over more conventional treatments while offering patients an effective, well-tolerated therapy option with generally low side effects. Moreover, mAbs are useful for biological and medical research.
Mature B-cells can be cultured in vitro under conditions that mimic some key aspects of the germinal centre (GC) reaction; that is, activation of B-cells with CD40 ligand (L) and the presence of cytokines like interleukin (IL)-4, IL-10 or IL-21. While B-cells cultured with CD40L, IL-2 and IL-4 produce very little Ig, addition of IL-21 leads to differentiation to plasma cells accompanied by high 1 g secretion. Although this in vitro system has proven useful to study some aspects of B-cell differentiation, both naive IgD+ B-cells and switched IgD-memory B-cells eventually differentiate into terminally differentiated plasma cells, which is accompanied by cell cycle arrest precluding the generation of long-term antigen-specific BCR-positive cell lines.
Recent advances have provided insight into how multiple transcription factors, including B-lymphocyte-induced maturation protein 1 (BLIMP1) and B-cell lymphoma (BCL) 6 control development of GC B-cells into terminally arrested, antibody-producing plasma cells. The transcriptional repressor BCL6 has been shown to prevent plasma cell differentiation. BCL6 is highly expressed in GC B-cells where it facilitates expansion of B-cells by down-regulating p53 and prevents premature differentiation of GC cells into plasma cells by negatively regulating BLIMP1.
An improved method for generating an antibody-producing plasmablast-like B-cell was recently described in WO 2007/067046, which is hereby incorporated by reference. According to this method, the amount of BCL6 and a Bcl-2 family member, preferably Bcl-xL, are modulated in a B-cell, preferably a memory B-cell, to generate an antibody-producing plasmablast-like B-cell. In WO 2007/067046, the amount of BCL6 and/or Bcl-xL expression product is either directly or indirectly influenced. Preferably, the amounts of both BCL6 and Bcl-xL expression products within the antibody-producing cell are increased, since both expression products are involved in the stability of an antibody-producing B-cell. Bcl-xL is a member of the anti-apoptotic Bcl-2 family. Processes that are controlled by the Bcl-2 family, which includes both pro- and anti-apoptotic proteins, relate to the mitochondrial pathway of apoptosis. This pathway proceeds when molecules sequestered between the outer and inner mitochondrial membranes are released into the cytosol by mitochondrial outer membrane permeabilization. The pro-apoptotic family members can be divided in two classes. The effector molecules Bax and Bak, which contain so-called Bcl-2 homology domain 3 (BH3) domains, are involved in permeablilizing the outer mitochondrial membrane by forming proteolipid pores; the pro-apoptotic BH3-only proteins (Bad, Bik, Bim, Bid, Hrk, Bmf, bNIP3, Puma and Noxa) function upon different cellular stresses by protein-protein interactions with other (anti-apoptotic) Bcl-2 family members.
Anti-apoptotic Bcl-2 family members Bcl-2, Bcl-xL, Bcl-w, A1 and Mcl-1 are generally integrated with the outer mitochondrial membrane. They directly bind and inhibit the pro-apoptotic Bcl-2 proteins to protect mitochondrial membrane integrity.
In such a method, it is further preferred that the antibody-producing plasmablast-like B-cell is incubated with IL 21 and CD40L. A B-cell, such as an antibody-producing plasmablast-like B-cell, is preferably cultured in the presence of CD40L since replication of most B-cells is favored by CD40L. It is furthermore preferred that STAT3 is activated in the antibody-producing B-cell. Activation of STAT3 can be achieved in a variety of ways. Preferably, STAT3 is activated by providing an antibody-producing cell with a cytokine. Cytokines, being naturally involved in B-cell differentiation, are very effective in regulating STAT proteins. Very effective activators of STAT3 are IL-2, IL-10, IL-21 and IL-6, but also IL-7, IL-9, IL-15, IL-23 and IL-27 are known to activate STAT3. Additionally, or alternatively, STAT3 activation is accomplished by transfer into a B-cell of a nucleic acid encoding a mutant of STAT3 that confers constitutive activation to STAT3. (Sean A. Diehl, Heike Schmidlin, Maho Nagasawa, Simon D. van Haren, Mark J. Kwakkenbos, Etsuko Yasuda, Tim Beaumont, Ferenc A. Scheeren, Hergen Spits STAT3-mediated up-regulation of BLIMP1 is coordinated with BCL6 down-regulation to control human plasma cell differentiation. J. Immunol. 2008 vol. 180 (7) pp. 4805-15.)
Most preferably, IL-21 is used, since IL-21 is particularly suitable for influencing the stability of an antibody producing plasmablast-like B-cell. In addition to up-regulating STAT3, IL-21 is capable of up-regulating BLIMP1 expression even when BLIMP1 expression is counteracted by BCL6. With the methods disclosed in WO 2007/067046, it has become possible to increase the replicative life span of an antibody-producing cell since it is possible to maintain a B-cell in a developmental stage wherein replication occurs. In earlier ex vivo B-cell cultures, the replicative life span was only a few weeks to two months. During this time, the cultured cells lose their capability of replicating and die. With a method as disclosed in WO 2007/067046, however, it has become possible to prolong the replicative life span of antibody-producing memory B-cells, so that ex vivo cultures are generated comprising plasmablast-like B-cells that are capable of replicating and producing antibody.
Although these methods enable the production of antibodies that efficiently target an antigen of interest, improvement of antibody characteristics, such as binding affinity, is often desired. Binding characteristics are, therefore, regularly altered by introducing mutations in the encoding nucleic acid, preferably in the CDR encoding region, and testing the resulting antibodies. This is, however, time consuming. Alternative methods for obtaining high affinity antibodies are, therefore, desired.
It is an object of the present invention to provide methods for producing and/or selecting high affinity antibodies.
The invention provides means and method for obtaining a B-cell population, starting from a given B-cell culture, which population has a higher average binding capacity than the original B-cell culture. Preferably, a monoclonal B-cell population is produced, starting from a monoclonal B-cell culture. Provided is a simple and elegant way of obtaining B-cell populations with an increased average binding capacity, without the need for laborious mutation techniques.
The invention provides a method for producing antibodies specific for an antigen of interest comprising:
a) selecting a B-cell capable of producing antibody specific for the antigen of interest or selecting a B-cell capable of differentiating into a B-cell that is capable of producing antibody specific for the antigen of interest;
b) inducing, enhancing and/or maintaining expression of BCL6 in the B-cell;
c) inducing, enhancing and/or maintaining expression of an anti-apoptotic nucleic acid in the B-cell;
d) allowing expansion of the B-cell into a population of the B-cells;
e) selecting at least one B-cell from the population of B-cells producing a B-cell receptor and/or antibody with a binding capacity higher than the average binding capacity of the population of B-cells for the antigen of interest;
f) culturing the at least one B-cell into a population of B-cells; and
g) obtaining antibodies produced by the B-cell culture.
Within a population of monoclonal B-cells capable of producing antibody specific for an antigen of interest, it is possible to select, in step e) of a method hereof, at least one, optionally more than one, such as, for instance, 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 25 or 50 B-cells with a binding capacity for the antigen of interest that is higher than the average binding capacity of the population of B-cells for the antigen of interest. Such B-cells with a higher binding capacity for an antigen of interest than the average binding capacity of the population of B-cells for the antigen of interest are herein also called “high-affinity B-cells.” One possible reason for a difference in binding capacity between multiple B-cells in a monoclonal population of B-cells is that the expression of the BCR varies between B-cells in the population. A B-cell with a relatively high expression of the BCR will bind more antigen of interest than a B-cell with a relatively low expression of the BCR. However, it is expected that antibodies produced by B-cells with different expression of the BCR have the same binding affinity. The present inventors surprisingly found that, besides a relatively high BCR expression, a collection of high-affinity B-cells produce antibodies specific for the antigen of interest that bind the antigen with a higher affinity than the average affinity of antibodies produced by the population of B-cells. Even more surprisingly, the inventors found that the B-cell cultures obtained with a method hereof contained cells that bound antigen with a higher affinity than the average B-cell in the original culture. Single B-cells can thus be isolated from a given B-cell population on the basis of their higher binding capacity by methods known in the art and be expanded to a new B-cell population in at least three weeks. These new B-cells produce antibodies that have a higher affinity than the antibodies produced by the original B-cell population that the new B-cells are derived from. This finding is contrary to expectations because a person skilled in the art would expect that after isolation of one B-cell (subclone) from an already monoclonal population of B-cells, the affinity for the antigen of antibody produced by the progeny of the subclone of the already monoclonal B-cell population will return to the average affinity for the antigen, comparable to the average affinity of the population of B-cells from which the at least one B-cell was selected.
Thus, in one embodiment in step a) of a method hereof, preferably a single B-cell is selected, for instance, from a polyclonal population of B-cells. The single B-cell is subsequently expanded into a monoclonal population of B-cells in steps b) to d). This is, for instance, achieved using a method as described in WO 2007/067046, which is discussed hereinbefore. Hence, in step d), a monoclonal B-cell line specific for an antigen of interest is obtained. In principle, all B-cells in the monoclonal B-cell line produce essentially the same antibodies specific for the antigen, although small differences in the affinity for the antigen may be present between cells of the monoclonal B-cell line, i.e., some B-cells in the monoclonal population produce antibodies with an affinity that is slightly higher than the average affinity and some B-cells in the monoclonal population produce antibodies with a slightly lower affinity. The population of B-cells becomes slightly heterogeneous again. In step e), at least one of such B-cells with a higher affinity than the average affinity is selected from the monoclonal B-cell line. In step f), the B-cell or B-cells selected in step e) are subsequently cultured into a second, preferably monoclonal, B-cell line. Provided is the insight that this second, preferably monoclonal, B-cell line has an average affinity that is higher than the average affinity of the original monoclonal B-cell population obtained in step d). As described above, it was surprisingly found that the high affinity of a selected B-cell is maintained after culturing, even if culturing takes place during a prolonged period of time, instead of returning to the average affinity of the original population. Thus, the second monoclonal population of B-cells cultured in step f) has a higher average affinity for the antigen than the monoclonal population of B-cells cultured in step d). Similarly, the affinity of most B-cells in the second monoclonal population of step f) is higher than the affinity of most B-cells in a monoclonal population of step d).
The invention thus provided in one embodiment is a method for obtaining a B-cell population with an increased average affinity for an antigen of interest, as compared to an original monoclonal B-cell population with a given average affinity for the antigen of interest, the method comprising:                providing a monoclonal B-cell population that is specific for the antigen of interest,        selecting at least one B-cell from the population of B-cells producing a B-cell receptor and/or antibody with a binding capacity higher than the average binding capacity of the population of B-cells for the antigen of interest; and        culturing the at least one B-cell into a population of B-cells.        
Further provided is a method for producing antibodies specific for an antigen of interest, the method comprising:
a) selecting a single B-cell capable of producing antibody specific for the antigen of interest or selecting a B-cell capable of differentiating into a B-cell that is capable of producing antibody specific for the antigen of interest;
b) inducing, enhancing and/or maintaining expression of BCL6 in the B-cell;
c) inducing, enhancing and/or maintaining expression of an anti-apoptotic nucleic acid in the B-cell;
d) allowing expansion of the B-cell into a first monoclonal B-cell line;
e) selecting from the first monoclonal B-cell line at least one B-cell that produces a B-cell receptor and/or antibody with a binding capacity for the antigen of interest higher than the average binding capacity of the first monoclonal B-cell line;
f) culturing the at least one B-cell selected in step e) into a second, preferably monoclonal, B-cell line; and
g) obtaining antibodies produced by the second, preferably monoclonal, B-cell line. Antibodies are obtained that have an affinity for the antigen of interest that is higher than the average affinity for the antigen of interest of antibodies produced by B-cells of the first monoclonal B-cell line.
In another embodiment, more than one B-cell is selected in step a) of a method hereof; for instance, 2, 3, 4, 5, 10, 15, 25, 50 or 100 B-cells. The B-cells are, for instance, selected from a polyclonal population of B-cells or from a biological sample. The selected B-cells are subsequently expanded into a population of B-cells in steps b) to d), for instance, using a method as described in WO 2007/067046. The obtained B-cell population is thus a (second) polyclonal B-cell population. Thereafter, and before step e) of a method hereof is carried out, a monoclonal population of B-cells is preferably produced. This is, for instance, done by selecting a single B-cell from the (second) polyclonal population of B-cells using Fluorescence Activated Cell Sorting or limiting dilution, which are explained hereinbelow, and expanding the selected single B-cell to a monoclonal population of B-cells. Then, step e) of a method hereof is carried out, in which at least one B-cell with a higher affinity than the average affinity of the monoclonal B-cell population is selected. In step f), the B-cell or B-cells selected in step e) are subsequently cultured into a second monoclonal B-cell line, after which, antibodies produced by the second monoclonal B-cell line are obtained in step g).
A method as described herein allows for obtaining improved, high-affinity antibodies, preferably monoclonal antibodies, without the use of recombinant techniques. Before the instant disclosure, affinity of (monoclonal) antibodies is increased using such recombinant techniques. The sequence of the nucleic acid encoding the antibody first needs to be determined. Subsequently, one or more mutations are introduced into the sequence of the nucleic acid encoding the antibody. Then, the genes containing one or more mutations need to be expressed in a cell followed by production of antibodies in producer cells. Finally, the mutated antibody has to be tested for its binding capacity to the antigen of interest in order to determine whether antibody with an improved affinity for the antigen as compared with the non-mutated antibody is obtained. Such a process for improving the affinity of an antibody is elaborate and time consuming. A method according to the instant disclosure allows the production of high-affinity antibody in a straight-forward and less elaborate process without the need of molecular engineering.
In one embodiment hereof, after the step of selecting at least one high-affinity B-cell from the already monoclonal population of B-cells (step e) of a method hereof as described above), the at least one high-affinity B-cell is allowed to expand into a population of B-cells, preferably a monoclonal B-cell line, again, after which another step of selecting at least one high-affinity B-cell from the new population of B-cells, preferably from the new monoclonal B-cell line, is performed. By repeating the steps of allowing expansion of a selected B-cell into a population and selecting at least one B-cell on the basis of its binding capacity for an antigen, i.e., repeating steps d) and e), it is possible to generate high-affinity antibody-producing B-cells. Preferably, by repeating the steps of expansion and selection as described above, it is possible to increase with each selection cycle the affinity of antibody produced by the resulting B-cell population for the antigen of interest.
A method is thus provided comprising, following step e) of a method hereof, repeating the step of allowing expansion of at least one selected high-affinity B-cell into a population of B-cells, preferably a monoclonal B-cell line, and selecting again at least one high-affinity B-cell, i.e., repeating steps d) and e) of a method hereof at least once. The steps are, for instance, repeated once, but preferably twice, three times, four times, five times or even more times.
In one embodiment, a method hereof is provided wherein the at least one B-cell selected in step e) is cultured for at least four weeks. Preferably, the at least one B-cell selected in step e) is cultured for at least six weeks, more preferably for at least nine weeks, more preferably for at least three months, more preferably for at least six months.
Without being bound to any theory, it is believed that differences in the affinity of antibodies for an antigen of interest within a population of monoclonal B-cells may result from processes mediated by Activation Induced Cytidine Deaminase (AID). Antigen-activated naive and memory B-cells in the germinal centre undergo extensive proliferation, accompanied by somatic hypermutations (SHM) and class-switch recombination (CSR) of 1 g genes mediated by AID. AID deaminates deoxycytidine residues in immunoglobulin genes, which triggers antibody diversification. It was demonstrated in patent application US2008305076 that IL 21 induces BLIMP, BCL6 and AID expression, but does not directly induce somatic hypermutation. However, the present inventors found that AID is expressed in B-cells that are cultured according to a method as herein described. The expression of AID in (a B-cell that will develop into) an antibody-producing B-cell allows the generation of novel immunoglobulins that harbor mutations that were not present in the original B-cell before transduction with BCL6 and an anti-apoptotic nucleic acid. Thus, culturing B-cells in which somatic hyper mutation is induced by expression of AID allows the generation of immunoglobulin variants that, for example, have a higher or lower affinity for an antigen of interest, or that are more stable, for example, in an aqueous solution or under increased salt conditions, or any combination thereof.
Upon selection of at least one high-affinity B-cell from the population of B-cells, AID is still expressed within the selected at least one B-cell. Therefore, after selection of such a B-cell, AID in the B-cell still allows the introduction of mutations in the immunoglobulin gene of the progeny of the B-cell. Somatic hypermutations in immunoglobulin genes occur preferentially in the CDR3 region of the Ig genes. Mutations introduced in the CDR3 region of the immunoglobulin are more likely to result in a reduced or lost binding affinity for an antigen of the immunoglobulin than in an increased binding affinity. The present inventors, however, did find increased binding affinity.
As used herein, the term “anti-apoptotic nucleic acid” refers to a nucleic acid that is capable of delaying and/or preventing apoptosis in a B-cell. Preferably, the anti-apoptotic nucleic acid is capable of delaying and/or preventing apoptosis in an antibody-producing B-cell. Preferably, an anti-apoptotic nucleic acid is used that comprises an exogenous nucleic acid. This means that either a nucleic acid sequence is used that is not naturally expressed in B-cells, or that an additional copy of a naturally occurring nucleic acid is used, so that expression in the resulting B-cells is enhanced as compared to natural B-cells. Various anti-apoptotic nucleic acids are known in the art, so that various embodiments are available. Preferably, a gene encoding an anti-apoptotic molecule is used. More preferably, a nucleic acid is used that is an anti-apoptotic member of the Bcl-2 family because anti-apoptotic Bcl-2 proteins are good apoptosis inhibiters. Many processes that are controlled by the Bcl-2 family (which family includes both pro- and antiapoptotic proteins) relate to the mitochondrial pathway of apoptosis, as outlined in more detail hereinbelow. Anti-apoptotic Bcl-2 family members Bcl-2, Bcl-xL, Bcl-w, A1 and Mcl-1 are preferred because they are generally integrated with the outer mitochondrial membrane. They directly bind and inhibit the pro-apoptotic proteins that belong to the Bcl-2 family to protect mitochondrial membrane integrity.
In a particularly preferred embodiment, the anti-apoptotic polynucleotide encodes Bcl-xL and/or Mcl-1 and/or a functional part of Bcl-xL and/or a functional part of Mcl-1. A combination of BCL6 and Bcl-xL nucleic acids, as well as a combination of BCL6 and Mcl-1 nucleic acids, is particularly suitable for immortalizing B-cells and long-term culture of the resulting plasmablast-like B-cells. Most preferably, the anti-apoptotic nucleic acid encodes Bcl-xL or a functional part thereof, because a combination of BCL6 and Bcl-xL stabilizes B-cells particularly well.
A functional part of Bcl-xL and a functional part of Mcl-1 are defined herein as fragments of Bcl-xL and Mcl-1, respectively, that have retained the same kind of anti-apoptotic characteristics as full-length Bcl-xL and Mcl-1, respectively, in kind (but not necessarily in amount). Functional fragments of Bcl-xL and Mcl-1 are typically shorter fragments of Bcl-xL and Mcl-1, which are capable of delaying and/or preventing apoptosis in a B-cell. Such functional fragments are, for instance, devoid of sequences that do not contribute to the anti-apoptotic activity of Bcl-xL or Mcl-1.
A population of B-cells hereof preferably is a monoclonal population of B-cells. An example of a population of B-cells hereof is a cell line of B-cells, preferably monoclonal B-cells. Hence, a population of B-cells hereof is most preferably a monoclonal B-cell line. Allowing expansion of the B-cell into a population of the B-cells is, for instance, accomplished by culturing the B-cell until a population of the B-cells is obtained.
Within a population of B-cells, even in a population of monoclonal B-cells, the binding capacity of the BCRs of the B-cells of the population, and the binding capacity of the antibodies produced by the B-cells of the population, is not equal. Instead, variation in the binding capacity exists. The average binding capacity of a population of B-cells is herein defined as the average of the binding capacity or average affinity of the BCR and/or antibody of all individual B-cells in the population. The average affinity for an antigen of interest of an antibody produced by a B-cell or by a population of B-cells is herein defined as the average of the affinities for the antigen of interest of the antibodies produced by all individual B-cells in the population. A high-affinity B-cell from a population of B-cells hereof, preferably from a monoclonal B-cell line, is preferably selected from the upper 40% of the B-cells of a population, preferably of a monoclonal B-cell line, with respect to binding capacity and/or affinity, preferably from the upper 30% of the B-cells of the population or monoclonal B-cell line, more preferably from the upper 25% of the B-cells of the population or monoclonal B-cell line, more preferably from the upper 20% of the B-cells of the population or monoclonal B-cell line, more preferably from the upper 15% of the B-cells of the population or monoclonal B-cell line, more preferably from the upper 10% of the B-cells of the population or monoclonal B-cell line, more preferably from the upper 1% of the B-cells of the population or monoclonal B-cell line. In one embodiment, one high-affinity B-cell is selected from the upper 1% of the B-cells of a population or monoclonal B-cell line with respect to binding capacity and/or affinity.
The average affinity for an antigen of interest of antibody produced by a population of B-cells, preferably by a monoclonal B-cell line, cultured from at least one high-affinity B-cell hereof is preferably at least 1.1 times the average affinity for the antigen of interest of the population of B-cells from which the at least one high-affinity B-cell was selected, more preferably at least 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 3.0, 3.5, 4.0, 5.0, 10.0, 20, 50, 100 times, or more, the average affinity for the antigen of interest.
The affinity of an antibody can be determined using any method known to a person skilled in the art. The affinity of an antibody is, for instance, determined using Enzyme-linked immunosorbent assay (ELISA), Surface Plasmon Resonance (such as Biacore) or Octet (ForteBio). Surface Plasmon Resonance (SPR) and Octet are techniques to measure biomolecular interactions in real-time in a label-free environment. For SPR, one of the interactants, for instance, an antibody, is immobilized to the sensor surface, the other, for instance, antigen, is free in solution and passed over the surface. Association and dissociation is measured in arbitrary units and preferably displayed in a sensorgram. Any change in the number of molecules bound to the biosensor tip causes a shift in the interference pattern that can be measured in real-time. Using Octet, the interference pattern of white light reflected from two surfaces, a layer of immobilized protein on the biosensor tip, and an internal reference layer is analyzed. The binding between a ligand immobilized on the biosensor tip surface, for instance, an antibody, and a protein in solution, for instance, an antigen of interest, produces an increase in optical thickness at the biosensor tip, which results in a wavelength shift that is a direct measure of the change in thickness of the biological layer. ELISA comprises immobilizing a protein, for instance, the antigen of interest, on the surface of the solid support, for example, a 96-well plate, and applying a sample to be detected or quantified on the solid support. Alternatively, a capture antibody is fixated on the surface of a solid support after which a sample containing the protein to be detected or quantified is applied to the immobilized capture antibody allowing the protein of interest to bind. Non-binding proteins are then washed away. Subsequently, a specific antibody conjugated to a label or an enzyme (or a primary antibody followed by a secondary antibody conjugated to a label or an enzyme) is added to the solid support. Preferably, the affinity constant (KD) of an antibody produced by a B-cell hereof is determined.
Binding of a B-cell hereof to an antigen of interest can be measured using any method known to a person skilled in the art. For instance, an antigen of interest is labeled with, for example, a fluorescent label. Detection of binding can subsequently be determined by various techniques, among which fluoresce microscopy and Fluorescence Activated Cell Sorting (FACS). FACS allows separation of cells in a suspension on the basis of size and the fluorescence of conjugated antibodies directed against surface antigens.
Selecting at least one high-affinity B-cell from a population of B-cells, preferably from a monoclonal B-cell line, can be performed using any method known to a person skilled in the art. Selection of at least one high-affinity B-cell hereof is, for instance, performed by cell sorting, for instance, using FACS (see above) or limited dilution. Limited dilution comprises the serial dilution of a suspension of cells, for instance, B-cells, until a single cell is present in a given volume. Subsequently, the binding capacity of each B-cell (after expansion of single cells to a population) is tested to allow selection of a B-cell-producing antibodies with a high affinity for antigen.
A B-cell capable of producing antibody is defined as a B-cell that is capable of producing and/or secreting antibody or a functional part thereof, and/or that is capable of developing into a cell that is capable of producing and/or secreting antibody or a functional part thereof.
A functional part of an antibody is defined as a part that has at least one same property as the antibody in kind, not necessarily in amount. The functional part is preferably capable of binding a same antigen as the antibody, albeit not necessarily to the same extent. A functional part of an antibody preferably comprises a single domain antibody, a single chain antibody, a FAB fragment, a nanobody, a unibody, a single chain variable fragment (scFv), or a F(ab′)2 fragment.
Non-limiting examples of a B-cell used or selected in a method hereof include B-cells derived from a human individual, rodent, rabbit, llama, pig, cow, goat, horse, ape, chimpanzee, macaque and gorilla. Preferably, the B-cell is a human cell, a murine cell, a rabbit cell, an ape cell, a chimpanzee cell, a macaque cell and/or a llama cell. Most preferably, the B-cell is a human B-cell.
In a preferred embodiment, a memory B-cell is selected in step a) of the method as described herein, for instance, a human memory B-cell. In a particularly preferred embodiment, the memory B-cell is a peripheral blood memory B-cell. Peripheral blood memory B-cells are easily obtained, without much discomfort for the individual from which they are derived, and appear to be very suitable for use in a method according to the instant disclosure.
A B-cell or a population of B-cells, preferably a monoclonal B-cell line, obtained with a method hereof is preferably stable for at least four weeks, more preferably at least six weeks, more preferably at least nine weeks, more preferably for at least three months, more preferably for at least six months, meaning that such B-cells are capable of both replicating and producing antibody, or capable of replicating and developing into a cell that produces antibody, during the time periods. B-cells hereof preferably comprise cells producing IgM or cells producing other immunoglobulin isotypes like IgG, or IgA, or IgE, preferably IgG. A B-cell hereof is particularly suitable for use in producing an antibody-producing cell line. High-affinity B-cells or a population or monoclonal B-cell line of high affinity B-cells hereof are preferably cultured ex vivo and antibody is preferably collected for further use. Antibodies or functional parts thereof produced with a method hereof are useful for a wide variety of applications, such as, for instance, therapeutic, prophylactic and diagnostic applications, as well as research purposes and ex vivo experiments. For instance, a screening assay is performed wherein antibodies or functional parts hereof are incubated with a sample in order to determine whether an antigen of interest is present.
In one embodiment, a high-affinity B-cell or a population or monoclonal B-cell line of high-affinity B-cells hereof comprises a human B-cell, capable of producing human antibody, because human antibodies are particularly suitable for therapeutic and/or prophylactic applications in human individuals.
The expression of BCL6 in a B-cell is induced, enhanced and/or maintained in a variety of ways. In one embodiment, a B-cell is provided with a nucleic acid encoding BCL6 or a homologue. In another embodiment, a B-cell is provided with a compound capable of directly or indirectly enhancing BCL6 expression. Such compound preferably comprises a Signal Transducer of Activation and Transcription 5 (STAT5) protein or a functional part, derivative and/or analogue thereof, and/or a nucleic acid sequence coding therefor. STAT5 is a signal transducer capable of enhancing BCL6 expression. There are two known forms of STAT5, STAT5a and STAT5b that are encoded by two different, tandemly linked genes. Administration and/or activation of STAT5 results in enhanced BCL6 levels. Hence, down-regulation of BCL6 by BLIMP1 is at least in part compensated by up-regulation of expression of BCL6 by STAT5 or a functional part, derivative and/or analogue thereof. Hence, STAT5 or a functional part, derivative and/or analogue thereof is capable of directly influencing BCL6 expression. It is also possible to indirectly influence BCL6 expression. This is, for instance, done by regulating the amount of a compound that, in turn, is capable of directly or indirectly activating STAT5 and/or regulating STAT5 expression. Hence, in one embodiment, the expression and/or activity of endogenous and/or exogenous STAT5 is increased. It is, for instance, possible to indirectly enhance BCL6 expression by culturing an antibody-producing cell in the presence of interleukin (IL) 2 and/or IL 4 or other cytokines that are capable of activating STAT5.
It is furthermore preferred that in a method hereof, the B-cells are at least at some stage incubated with IL 21 and CD40L. A B-cell, such as an antibody-producing plasmablast-like B-cell, is preferably cultured in the presence of CD40L since replication of most B-cells is favored by CD40L. It is furthermore preferred that STAT3 is activated in the B-cell. Most preferably, IL 21 is used for up-regulating STAT3, since IL 21 is particularly suitable for influencing the stability of a B-cell hereof. In addition to up-regulating STAT3, IL 21 is capable of up-regulating BLIMP1 expression even when BLIMP1 expression is counteracted by BCL6.
In another embodiment, the amount of BLIMP1 expression product in the B-cell selected in step a) of a method hereof is directly or indirectly controlled. The amount of BLIMP1 expression product can be controlled in various ways, for instance, by regulating STAT3 or a functional part, derivative or analogue thereof STAT3 is activated in a variety of ways. Preferably, STAT3 is activated by providing a B-cell hereof with a cytokine. Cytokines, being naturally involved in B-cell differentiation, are very effective in regulating STAT proteins. Very effective activators of STAT3 are IL-21 and IL-6, but also IL-2, IL-7, IL-10, IL-15 and IL-27 are known to activate STAT3. Moreover, Toll-like receptors (TLRs) that are involved in innate immunity are also capable of activating STAT3. Most preferably, IL-21 is used. IL-21 is capable of up-regulating BLIMP1 expression even when BLIMP1 expression is counteracted by BCL6.
By “a functional part of STAT5 or STAT3” is meant a proteinaceous molecule that has the same capability—in kind, not necessarily in amount—of influencing the stability of an antibody-producing cell as compared to STAT5 or STAT3, respectively. A functional part of a STAT5 protein or a STAT3 protein is, for instance, devoid of amino acids that are not, or only very little, involved in said capability. A derivative of STAT5 or STAT3 is defined as a protein that has been altered such that the capability of the protein of influencing the stability of an antibody-producing cell is essentially the same in kind, not necessarily in amount. A derivative is provided in many ways, for instance, through conservative amino acid substitution wherein one amino acid is substituted by another amino acid with generally similar properties (size, hydrophobicity, etc.), such that the overall functioning is likely not to be seriously affected. A derivative, for instance, comprises a fusion protein, such as a STAT5-ER fusion protein whose activity depends on the presence of 4 hydroxy-tamoxifen (4HT). An analogue of STAT5 or STAT3 is defined as a molecule having the same capability of influencing the stability of an antibody-producing cell in kind, not necessarily in amount. The analogue is not necessarily derived from the STAT5 or STAT3 protein.
A method hereof is preferably used for generating a cell line of high-affinity B-cells that is stable for at least one week, preferably at least one month, more preferably at least three months, more preferably at least six months so that commercial high-affinity antibody production has become possible. Preferably, a stable cell line capable of producing monoclonal high-affinity antibodies is produced. This is preferably performed by using memory B-cells that have, for instance, been isolated from a sample by selection for CD19 (B-cell marker) and cell surface IgG and/or CD27 (to mark memory cells). Furthermore, a memory B-cell capable of specifically binding an antigen of interest is, for instance, selected in a binding assay using the antigen of interest. Subsequently, BCL6 and an anti-apoptotic nucleic acid, preferably Bcl-XL or Mcl-1, are preferably co-expressed in the B-cell, resulting in a population of cells specific for the antigen of interest. Preferably, only one memory cell is selected in step a) of a method as described herein, so that a B-cell population hereof producing monoclonal antibodies (a monoclonal B-cell line) is obtained.
In one embodiment, a B-cell, preferably, but not necessarily, a memory B-cell, that originates from an individual that had been previously exposed to an antigen of interest, is used in a method hereof. However, this is not necessary. It is also possible to use a B-cell from an individual that has not been exposed to the antigen of interest. For instance, a B-cell is used that is specific for another antigen but shows cross-reactivity with the antigen of interest. As another example, a B-cell is used that is selected from a naive B-cell population of an individual. The naive B-cell population of an individual may contain B-cells that show reactivity with an antigen of interest even though the individual has not been exposed to the antigen of interest. Such B-cell from a naive B-cell population is, for instance, selected using labeled antigen of interest.
The invention furthermore provided are isolated or recombinant B-cells and populations of B-cells, preferably monoclonal B-cell lines, obtained by a method hereof. Such high-affinity B-cells are preferably stable for at least one week, preferably for at least one month, more preferably for at least three months, more preferably for at least six months, meaning that the B-cell is capable of both replicating and producing antibody, or capable of replicating and developing into a cell that produces antibody, during the time periods. B-cells hereof preferably comprise cells producing IgM or cells producing other immunoglobulin isotypes like IgG, or IgA, or IgE, preferably IgG. A B-cell hereof is particularly suitable for use in producing an antibody-producing cell line. High-affinity B-cells hereof are preferably cultured ex vivo and antibody is preferably collected for further use. Antibodies obtained from a B-cell or from a B-cell population or monoclonal cell line hereof are also provided. High-affinity antibodies or functional parts thereof produced with a method hereof are useful for a wide variety of applications, such as, for instance, therapeutic, prophylactic and diagnostic applications, as well as research purposes and ex vivo experiments. For instance, a screening assay is performed wherein antibodies or functional parts hereof are incubated with a sample in order to determine whether an antigen of interest is present.
B-cells generated with a method hereof are particularly suitable for producing high-affinity antibodies against an antigen of interest. In one preferred embodiment, however, the genes encoding the Ig heavy and/or light chains are isolated from the cell and expressed in a second cell, such as, for instance, cells of a Chinese hamster ovary (CHO) cell line. The second cell, also called herein a “producer cell,” is preferably adapted to commercial antibody production. Proliferation of the producer cell results in a producer cell line capable of producing antibody. Preferably, the producer cell line is suitable for producing compounds for use in humans. Hence, the producer cell line is preferably free of pathogenic agents such as pathogenic micro-organisms.
The invention is further explained by the following, non-limiting, examples.